Glycerol is a chemical, 1,2,3-propanetriol, which may be obtained either by chemical synthesis from propylene, or as a by-product formed during the methanolysis of vegetable oils.
The methanolysis of vegetable oils may be carried out according to various processes, in particular by using a homogeneous catalyst such as sodium hydroxide or sodium methylate in solution in methanol, or by using a heterogenous catalyst. Reference may be made, on this subject, to the article by D. Ballerini et al. in L'Actualité Chimique of November-December 2002.
The methanolysis of vegetable oils results, on the one hand, in methyl esters, and on the other hand in glycerol. Methyl esters are used in particular as fuels or combustibles in diesel fuel and domestic fuel. With the development of fuels of renewable origins, and especially of vegetable oil methyl esters (VOMEs), the production of glycerol in accordance with this production method increases greatly, glycerol representing around 10% by weight of the oil converted.
Glycerol derived from vegetable oils is a natural product of renewable origin which is thus increasingly available. In the current context of the novel concept of green chemistry, and more generally of sustainable development, it is becoming increasingly advantageous to utilize this product.
However, the methods for producing VOMEs result in a glycerol that is more or less pure and more or less dilute in water. Generally, it is these more or less pure aqueous solutions of glycerol that are known as glycerin, according to the definition adopted by “The Soap and Detergent Association” (Soaps and Detergents: A theoretical and Practical Review, Miami Beach Fla., Oct. 12-14, 1994, chapter 6 pp. 172-206. Ed: L Spitz, AOCS Press, Champaign). Crude glycerin generally has a composition of the order of 88% glycerol, 9-10% water and 2-3% impurities. In particular, it may contain impurities such as basic salts (for example of sodium or of potassium), non-glycerin organic compounds, methanol or residues of vegetables oils. In certain applications of glycerol, the presence of these impurities is particularly prejudicial for the reactions carried out or for the quality of the finished products. For example, in the case of the production of acrolein, the presence of sodium or potassium salts is prejudicial for the catalytic dehydration reaction of glycerol to acrolein, as these salts are capable of poisoning the acid sites of the catalysts used.
Consequently, the aqueous solutions of crude glycerol or glycerin generally require a pretreatment before use, or a purification treatment in order to envisage novel applications.
Furthermore, it is often necessary not only to remove the impurities that are undesirable for the envisaged application, but also to concentrate the aqueous solution, or even to vaporize the aqueous solution, certain industrial processes using glycerol in vapor form. These operations are tricky as it is known that glycerol may decompose, in particular to acrolein, or result in polymers such as polyglycerol.
Various technologies for purifying glycerol have been described in the literature. Specifically, it is a product that has more than 1500 different applications, all requiring particular qualities, in particular there is a “Pharmacopeia” grade that requires a high purity of the glycerol.
Among the methods used or studied for the purification and evaporation of glycerol, mention will especially be made of those that are described by G. B. D'Souza, in J. Am. Oil Chemists' Soc. November 1979 (Vol 56) 812A, by Steinberner U et al., in Fat. Sci. Technol. (1987), 89 Jahrgang No. 8, pp. 297-303, and by Anderson D. D. et al. in Soaps and Detergents: A theoretical and Practical Review, Miami Beach Fla., Oct. 12-14, 1994, chapter 6, pp. 172-206. Ed: L Spitz, AOCS Press, Champaign.
The treatments of crude glycerol solutions proposed target the removal of the dissolved salts and of the organic impurities resulting from fatty substances, the removal of the color, an increase in the glycerol content, or the vaporization of the glycerol, depending on the final application envisaged.
In particular, in order to achieve these objectives, an evaporation, a distillation, a treatment with lime (in order to neutralize the residual fatty acids) followed by a filtration, an ion exchange or ion exclusion treatment, a separation by reverse osmosis or an electrodialysis may be carried out.
Multiple-effect evaporators are, for example, used for concentrating dilute solutions of glycerol. With a triple-effect evaporator, it is thus possible to evaporate 2.4 kg of water with 1 kg of steam.
Distillation is one of the techniques used for concentrating and purifying glycerin. As glycerol begins to decompose at around 202° C., i.e. well below its boiling point (293° C.), it is necessary to distil glycerin in several steps using reduced pressure. In certain cases, the distillation is carried out via batch operations, until the salts and the non-volatile compounds have accumulated sufficiently in the vessel. The operation is then stopped and the impurities are discharged from the vessel before restarting the distillation. The evaporation is carried out under vacuum, and the partial condensation of glycerol (which will condense before water) at the outlet of the unit makes it possible to directly obtain a concentrated glycerol. Typically, pressures of 10 mm Hg are used, for a temperature of 160-165° C., which gives low partial pressures of glycerol in the vapor phase.
The distilled glycerin still contains colored compounds. It is sometimes necessary to decolorize the glycerin for pharmaceutical and food applications. Typically activated carbon is added to the glycerin in order to decolorize it.
The purification of glycerin by ion exclusion has also been developed and uses ion resins in order to separate the ionic salts that are soluble in aqueous solution from non-ionic compounds such as glycerol. This is a technique which avoids the consumption of heat and of chemical regenerants, and which makes it possible to purify highly contaminated streams such as crude glycerin, using only water as a chemical regenerant.
Aqueous solutions of glycerol that are weakly contaminated by salts may be exchanged simply over acid and basic resins. The thus purified glycerol solutions may then be concentrated by evaporation.
The technique of reverse osmosis, based on a separation over a semi-permeable membrane by applying a pressure has been proposed for the concentration of particularly dilute glycerol streams.
Solutions of glycerin and of sodium hydroxide in methanol obtained after transesterification of rapeseed oil have been demineralized by membrane electrodialysis to produce pure glycerin. This technique is described in the reference: Schaffner, F. et al., Proc.—World Filtr. Congr. 7th, Volume 2, 629-633.
In the methods proposed for evaporating aqueous solutions of glycerol, the control of the temperature is very critical as certain undesirable reactions may take place, such as the formation of nitrogen-containing compounds by a degradation of protein matter present in the glycerin, the formation of a volatile glycerin ester by reaction with soaps of low molar mass, the formation of polyglycerol, the formation of acrolein which contributes to the odors of the final product. It is therefore important to limit the residence time of the glycerin at high temperature, and also this temperature. The evaporation processes used conventionally do not therefore make it possible to have high partial pressures of glycerol in the vapor phase. Furthermore, it is often necessary to combine several treatments in order to obtain the glycerol with a purity and at a concentration that are suitable for the envisaged application.